Lrp (leucine-responsive regulatory protein), usually together with leucine, influences the expression of dozens of operons of Escherichia coli. One of the most interesting aspects of the Lrp regulon is the number of different patterns of regulation that are caused by the interaction of Lrp and leucine. Lrp activates some operons and represses others. For each of these two categories, examples exist in which 1) leucine prevents the action of Lrp, 2) leucine is required for the action of Lrp, and 3) leucine has no effect on the action of Lrp. A major focus of this proposal is to understand how a simple regulatory protein in combination with a ligand can have so many different effects upon gene expression. The specific aims of this proposal are to carry out a basic characterization of Lrp and of its interactions with DNA and leucine, an investigation of the mechanisms by which Lrp and leucine interact to produce so many different regulatory patterns, a study of the mechanism by which Lrp activates transcription from a particular promoter (the promoter of the ilvIH operon), an investigation of lrp gene regulation aimed at understanding how the Lrp regulon interacts with other regulons, and a broad study probing the distribution of Lrp-like regulons in nature. The proposed investigations will involve biophysical, biochemical, molecular biological, and genetic methodologies. One aspect of Lrp function has some relevance to pathogenicity of some groups of microorganisms. Lrp is required for expression of a number of operons involved in the formation of pili, appendages that allow bacterial to colonize regions of the body such as the urinary track. Basic studies of Lrp structure and function will contribute to our understanding of some forms of microbial pathogenesis.